Shifts in climate lead to greater violence and conflict, according to a wide-ranging analysis in Science that draws upon data from dozens of studies.

The researchers gathered studies on the topic from a variety of academic disciplines, including archaeology, psychology, economics, and criminology. They chose data sets that could convincingly suggest a causal link. For example, studies that simply compared two countries’ levels of conflict were not included, since many factors besides climate could have accounted for the differences. Instead, the team tried to find studies that tracked levels of conflict within the same country as climate changed.

The authors ended up with 60 studies from around the world, which used data going back as far as 10,000 B.C.E. The patterns revealed in these data sets were “remarkably similar,” the team writes. For example, studies have shown that people become more aggressive in warmer conditions, and Hindu-Muslim riots are more common after periods of unusually rainy weather. The link between climate and conflict held true “across types of conflict, across human history, across regions of the world, across income groups,” the authors write.

The team estimates that when temperature or rainfall rises by one standard deviation, violence between individual people increases by 4 percent and conflict between groups by 14 percent. Climate change might trigger conflict because it lowers economic productivity, increases inequality, or prompts large-scale migrations. — Roberta Kwok | 1 August 2013